Accretion discs

Accretion discs are circular clumps of matter/dust that orbit a central body, which can be a young star, a neutron star, a white dwarf or even a black hole. In these central bodies, the occurrence of accretion discs around dense objects is notable. Moreover, such discs are commonly formed in binary star systems (two-star systems) with the presence of a white dwarf orbiting a common baricenter/center of mass. As the gravitational interactions of such a system happen, the white dwarf “steals” the mass of its neighbor so that the in going matter forms a disk around the thief. The matter being transferred enters an unstable state and an angular momentum redistribution of the disc occurs and causes the gas to spiral toward the thief star and thus there is an increase in the central body mass. If the mass gain is significant, it is possible that the white dwarf will cross the Chandrasekhar boundary, causing the star to collapse, resulting in a type I supernova.

Accretion discs are widely used to study the behavior of celestial bodies due to the high emission of electromagnetic radiation, which facilitates the detection and observation. As the gas follows its spiral path until it is "devoured", gravitational energy is transformed into thermal energy, radiating onto the disk surface as electromagnetic radiation (frequency varies depending which central body is considered). In the case of black holes, as seen in the recent Messier 87 galaxy supermassive blackhole photo, the released radiation is highly energetic, lying in the X-ray range. Also remarkable is the incredible phenomenon of relativistic jets, plasma jets coming from galactic nuclei (such as quasars), which are theorized to be formed from complicated interactions of the magnetic fields in the accretion discs. The term “relativistic” indicates that matter around the central body is being accelerated at speeds close to light (99.995% of the speed of light) and thus exhibits relativistic effects, described by Albert Einstein's special theory of relativity.

In addition to the most extreme contexts of the cosmos, the presence of accretion discs is also given in the formation of planetary systems, such as the solar system, in such situations the cluster of surrounding matter is called protoplanetary disk, which is responsible for the formation of planets.

Reference Material: https://svs.gsfc.nasa.gov/10139

Fundamentals of Astronomy (Gregorio-Hetem & Jatenco-Pereira)

Death by Black Hole (Neil deGrasse Tyson)

50 Astronomy Ideas You Need to Know (Helena Londres)

The photos below illustrate the accretion discs, the last one is the picture obtained from the supermassive black hole of Messier 87.